1. Field of the Invention
The present invention relates to a printer and, more particularly, to the improvement of the performance of a printer which is usable as an electronic copying machine or a non-impact page printer.
2. Description of the Prior Art
Printing apparatus which are usable as an electronic copying machine or a non-impact pace printer have hitherto adopted various systems such as an electrophotography system, ionography, magetography and electrostatic photography.
In any of these systems, a latent image is first formed electrostatically or magnetically on a photosensitive material. Toner is then attracted to the latent image by electrostatic power, magnetic attractive force or the like, thereby developing the image. The toner image s transferred to a recording medium such as paper and film fix by heat and finally printed.
A first example of conventional printers will now be explained with reference to FIG. 8. A printer of this type is disclosed in Japanese Patent Laid-Open No. 194472/1985.
This kind of printer is a non-impact page printer of an electrophotography system. Input signals such as a signal concerning an image to be printed and a signal input from another data output device are converted to a signal for controlling a laser write unit 12 by a signal processor 10. On the basis of the thus-converted signal, the laser write unit 12 writes a latent image on a photosensitive material, namely, a photosensitive drum 14. Since the photosensitive drum 14 is uniformly charged by an electrifier 16, when the written image is exposed to the laser spot of the laser write unit 12, a predetermined latent image is formed on the surface of the photosensitive drum 14.
The developing unit 18 causes toner to electrostatically adhere to the latent image to form a toner image. A transfer device 22 transfers the toner image to a recording medium, namely, recording paper 20 which is fed in synchronization with the photosensitive drum 14.
The recording paper 20 with the toner image transferred thereto by the transfer device 22 is separated from the surface of the photosensitive drum 14 by a separator 24. After toner image is transferred from the surface of the photosensitive drum 14, the electrostatic charges are eliminated from the surface by a static eliminator 26 and the toner remaining on the surface is removed by a cleaner 28. Thus, the photosensitive drum 14 is ready for next printing.
The toner image transferred to the recording paper 20 is fixed on the surface of the recording paper 20 by a fixing device 30. The fixing device 30 is provided with a heat roller 32, for heating the toner on the recording paper 20 to fix it on the recording paper 20, and a press roller 34 which is provided so as to face the heat roller 32 and press the recording paper 20 against the heat roller 32.
The heat roller 32 is provided with a heater 36 therewithin. The header 36 is so controlled as to have a predetermined surface temperature on the basis of the detected value of a sensor 38 for detecting the surface temperature of the heat roller 32. Felt 40 is provided so as to clean the heat roller 32 and prevent the fusion of the recording paper 20.
A second example of conventional printers such as that shown in FIG. 9 is disclosed in U.S. Pat. No. 4,207,101.
In this apparatus, a photosensitive belt 42 with a toner image formed thereon comes into contact with an intermediate roller 44 while the photosensitive belt 42 travels along the periphery of a support roller 42a. A bar magnet 44a is provided within the support roller 42a so as to face the support roller 42a. The intermediate roller 44 has a portion consisting of countless magnetic materials which is divided from a part consisting of a non-magnetic material. Therefore, the part consisting of the magnetic materials is magnetized by the bar magnet 44a, thereby transferring the toner image which is formed by the toner containing a magnetic material onto the intermediate roller 44.
A lamp 44b irradiates the entire part of the photosensitive belt 42 so as to enhance the efficiency of transferring the toner image to the intermediate roller 44. The toner image transferred to the intermediate roller 44 is then transferred onto the recording paper 20 by pressing the intermediate roller 44 against the recording paper 20 by means of a press roller 46. The toner image is fixed on the recording paper 20 by a heating fixer 48.
These conventional printers, however, have the following problems.
In the first example, a high voltage must be applied to the transfer device 22 in order to transfer the toner image formed on the photosensitive drum 14 by electrostatic attraction of the toner to the recording paper 20. When the recording paper 20 is separated from the surface of the photosensitive drum 14, application of a considerably high voltage is also required. When such a high voltage is applied, discharge is sometimes produced depending upon the ambient environmental conditions such as humidity. If such discharge is produced, not only the photosensitive drum 14 is deteriorated but also ozone harmful to human body is produced.
Since toner powders have the same electric charge, repulsive force is applied between the toner powders, thereby diffusing and blurring the image at the time of transfer.
In addition, the toner image transferred onto the recording paper 20 has not been fixed, so that when external force such as vibration an shock is applied, the toner image gets out of shape. It is therefore necessary to keep the recording paper 20 out of touch with the surface having the toner image force facing upward so as to retain the toner image by gravity until the toner image is fixed on the recording paper 20. It is also necessary to protect the toner image from external force such as air flow until it has been stabilized.
Since the electrostatic attractive force is microscopically strong, the ratio at which the toner is transferred from the photosensitive drum 14 is at most 70 to 80% in this system, and the residual toner remains on the photosensitive drum 14. It is therefore necessary to remove the residual toner from the photosensitive drum 14 by the cleaner 28.
As a result, the following problems are brought about such as that the printing density becomes low, that the efficiency of the toner is lowered, that the mechanism for accommodating disposed toner becomes complicated, and that a space is required for collecting and accommodating toner periodically.
The toner image is fixed on the recording paper 20 by heating the recording paper 20 together with the toner to a temperature higher than the melting point of the toner. For this purpose, electric power as large as 10 W to several KW is required. Such power is too large for an apparatus for domestic use.
When the recording paper 20 is heated, there are cases where the quality of the paper is deteriorated, the color is changed, the strength is lowered, and the paper is curled.
Since no external force is to be applied to the recording paper 20 until the toner image is fixed, the course through which the recording paper 20 passes is set at a position in the apparatus which is not affected by external force, in other words, at the inner part. Therefore, jamming is likely to be produced during the travel of the recording paper 20 and such an inner part is inconvenient for recovery operation at the time of production of jamming, while a structure for facilitating the recovery operation is very complicated and expensive.
In the second example of the conventional printers, since magnetic force is utilized for transferring the toner image on the photosensitive belt 42 to surface of the intermediate drum 44, it is difficult to harmonize the transfer of the toner image from the photosensitive belt 42 to the intermediate drum 44 with the transfer of the toner image from the intermediate drum 44 to the recording paper 20. More specifically, if the toner image is completely transferred to the surface of the intermediate drum 44, the toner firmly adheres to the surface of the intermediate drum 44, thereby making it difficult to transfer the toner image to the recording paper 20.
On the other hand, if the adhesion between the toner and the surface of the intermediate drum 44 is weak, the transfer from the photosensitive belt 42 to the intermediate drum 44 becomes insecure, and the toner image disadvantageously gets out of shape on the intermediate drum 44.
This apparatus is also restricted in design because no external force must be applied to the recording paper 20 to which the toner image has been transferred from the surface of the intermediate drum 44 until it has been fixed on the recording paper 20.